“This afternoon we’re going to make poo stew,” says Teresa Cross. Her announcement is greeted with giggles and a chorus of “eurrrgh” from the class of Year 5s. Whatever it is about children and their fascination with poo and wee, pupils in this class are about to see the real thing. Lots and lots of it.

Hayden Education Centre is based at a sewage works in Cheltenham, Gloucestershire run by Severn Trent Water. Forty million litres of foul effluent pours in there each day, to be filtered, sifted, dined on by microbes and cleaned up.

But does sewage and what happens to it really make for a good school trip? You bet it does. If you can stand the smell, it is strangely compelling.

Pupils from The Grove Junior School in Malvern are certainly hooked. The class recently attended one of the centre’s water days, which involve a range of hands-on classroom activities and a tour of the sewage works. The activities are designed to fit in with the national curriculum key stages for geography, science and citizenship. They are delivered by Teresa Cross, a teacher from Steam Mills Primary School in the Forest of Dean, who is on secondment to Severn Trent Water to teach at the centre.

Teresa’s delivery is very enthusiastic. She starts by getting pupils to write down what they know about water. They talk about its different states - gas, liquid and solid - and about the water cycle. How old is water? One bright spark in the class puts up his hand and says it’s older than the dinosaurs. “Very good - it’s four and a half billion years old,” says Teresa. “I think I’m drinking water that’s been through a caveman.” Then a picture of England and Manchester United striker Wayne Rooney appears on the whiteboard. “It’s possible I might have drunk water that’s been through him,” she says. “Eurrgh!” groan the children.

They discuss the destructive power of water. Pictures flash up on the whiteboard of the aftermath of the Boscastle flood, the Boxing Day tsunami and flooding in New Orleans in the wake of Hurricane Katrina. There are important messages here too. Teresa shows how much water we waste by brushing her teeth and leaving the tap running to fill a bucket. Then she enlists a volunteer to see how far she can carry the heavy bucket around the building. Showing photos of a school in Kenya, she explains that most of its pupils’ families have to walk miles each day to get water from a river. Then, after a brief safety talk, coats and hats go on for a tour of the works. Teresa and education manager Derek Allder walk the group through the whole water treatment process.

The journey starts at the inlet, where effluent pours into the site and the stench is truly foul. Holding scarves over their noses, the pupils are encouraged to look in. As well as all the obvious, some surprising things come through here, like toys and dead goldfish. “Four weeks ago a man rang up asking for his false teeth back,” says Teresa, showing them the screen house where rubbish is filtered out. “I’m afraid we didn’t find them.”

The journey moves on through different stages. There are grit tanks to remove the gravel and sand, and settling tanks where a rotating arm skims off the fat. The next stage Teresa calls “Hayden’s little restaurant” - huge tanks where oxygen is bubbled through the remaining sewage and bacteria clean it up. After further settling and filtering processes, all the solids end up in a huge silo where they are made into compost; methane is converted into electricity and the water left over is clean enough to be channelled back into the River Chelt.

Back at the education centre, the children get stuck into making “poo stew”, as promised. Each group adds a list of ingredients to water that simulate the constituents of sewage. They also get a chance to look under the microscope at the microbes used in the process.

Severn Trent has five purpose-built education centres; as well as Cheltenham, there are centres in Nottingham, Leicester, Solihull and Carsington in Derbyshire. Last year 22,000 children visited free of charge, learning about the water cycle, global water issues and waste. Teresa Cross makes no apologies for appealing to children’s scatological side. “I’m afraid there is a lot of that. And that’s the way it should be, because we want the kids to relax and feel comfortable about it. Because we’re literally talking about crap, and we want the children to know that every time they go to the loo they are delivering something to us that we can do something with. That’s what this tour is about - they can see that this is a journey.”

* www.stwater.co.uklearning

* Unesco has designated March 22 as World Water Day 2006 www.unesco.orgwaterwwd2006index.shtml